Fire blight is a plant disease caused by the bacterium Erwinia amylovora, and is the cause of massive commercial damage worldwide, especially in the field of pomiculture. The disease is spreads rapidly and is difficult to treat and control with current agents.
Known signs for the manifestation of the disease are the dark-brown to black leaf stalks of leaves and blossoms already dead located in the proximity of diseased branches as well as the blackened midrips of the leaves. During spring and summer, sticky, moist droplets appear on the affected sprouts, fruits and on the downside of the leaves, these droplets being colorless at first but later on assume a brownish color. The infectious oozes (the bacterial exudates) preferably form under hot and humid conditions.
Traditionally, for treating fire blight already present, affected parts of the plant are removed and burned or plant protection agents containing antibiotics, e.g. streptomycin, are employed comprehensively. While burning is only marginally successful and the recurrence of disease can be prevented only if all affected plant material is completely deposited, the treatment with antibiotics is problematic for a variety of reasons. From a food technology perspective, the use of antibiotics in orchards requires elaborate testing for residual antibiotics in the treated plants as well as in the fruits produced by these trees, to ensure, in the interest of the consumer, that the fruit itself is free from antibiotics. Moreover, the development of antibiotic resistances in bacterial strains is a risk, occurring e.g. when antibiotics are applied inexpertly for extended periods of time at too low concentrations.
Furthermore, the use of plant protecting products containing antibiotics in orchards localized in the vicinity of a honey producing apicultural facility is problematic as well. Antibiotics may be taken up by worker bees and appear as undesired antibiotic residues in the honey produced by those bees.
From the state of the art, some agents suitable for treating and preventing fire blight are known which are free from antibiotics and work more or less effective. The prior art documents WO 2005/1048717, CA 2291984 and U.S. Pat. No. 4,569,841 disclose biotechnological or fermentation-based methods for treating fire blight. Finally, the documents WO 2002/1052942 and EP 0 565 266 disclose compositions containing inorganic substances for treating fire blight.
WO 2005/099454 relates to combinations of active substances which contain a valinamide derivative, a phosphonate and folpet and which can be used for combating phytopathogenic fungi and bacteria. Compositions used in the examples contain fosetyl-Al (Aliette) as an active ingredient, in which aluminum is bound to an ethylphosphonate residue. The concentration of the fosetyl-aluminum in these compositions, however, amounts to a maximum of only 250 mg/L (see Table 1 of this disclosure), which corresponds to an aluminum concentration of about 0.7 mmol/L. The principle of action of Fosetyl-Al is based on a systemic transformation into phosphonic acid and on an enhancement of the plant's resistance against harmful fungal and bacterial organisms. Additionally, in the prior art document WO 2005/099454, no pH value of the compositions is given.
Furthermore, the documents DD-A 273 192, JP-A 630099005 and GB-A 1315430 disclose methods of treatment and prevention by using known antibiotics. The documents GB 1049116, JP 1090102, U.S. Pat. No. 5,686,389, EP 1 075 185, EP 0 158 074, EP 1 300 078 and DE 3640048 disclose other organic compounds effective against fire blight.
None of the cited prior art documents, however, discloses a composition characterized by an excellent efficiency with respect to treating and preventing fire blight, while at the same time being ecologically safe and non-harmful from a health point of view in the context of food production. Moreover, none of the documents discloses a composition which contains aluminum (III) ions at the concentrations and at the specific pH value as defined in the patent claims.